Various applications require the delivery of chemicals at accurately metered low flow rates. One such application is the dispensing or injecting of small amounts of drying agents into the rinse water used in commercial or institutional dish washing apparatus. In such applications the feed rate must often be adjusted down to a very low value. At such low flow rates, ability to prime is very poor. An accurate metering, low volume pump requires much time just to exhaust the air from a dry suction line. Now this time is increased by:
1. Volume of air in the suction line (length and size of line). PA0 2. Differential air pressure required to open the check valves. PA0 3. Height of the metering pump over the liquid source. With increasing height the small metering pump must exhaust a greater volume of air since the air is at reduced pressure. PA0 4. Altitude which has the same effect as 3 (above). PA0 5. Volatility of liquid, since this can require the pump to exhaust vapor as well as air. PA0 6. Temperature of the liquid either in the container or at any area in the suction line and pump, since this always raises the volatility.
Whenever one or more of these factors are present, the small metering valve primes slowly or not at 11 resulting in failure of injection of the chemical into the host medium with attendant process failures. U.S. Pat. No. 3,680,985 discloses a prior art attempt to deal with this problem.
The apparatus of the present invention utilizes a first pump whose capacity is substantially in excess of the low flow rate of the fluid to be injected into the host medium. This larger capacity pump insures that there will be a minimum priming delay as the fluid is drawn from the remote supply. The pump discharges into a small separation chamber or priming reservoir which has an overflow line leading back to the fluid supply. A second pump, having a much smaller, but adjustable, flow rate draws fluid from this separation chamber and discharges it, to the injection line which delivers the fluid to the host medium. This separation chamber is so designed that air and vapor bubbles delivered to it rise with the excess liquid and pass thru to the overflow line so they do not adversely effect the rate of the metering pump.